THE PRESENT INVENTION relates to a shielding device and more particularly, relates to a shielding device for use with a portable communication unit such as a paging unit and especially a cellular or radio telephone.
In recent years there has been a substantial increase in the number of communication units in the form of paging units and cellular or radio telephones that are in use. Paging units comprise totally passive devices which simply act to receive signals transmitted from a central transmitter and subsequently to provide a visual or audible indication that such a signal has been received, or active paging devices which can be used not only to receive signals from a central transmitter, but also to transmit signals to the central transmitter to provide, for example, an indication of the approximately area in which the paging device is located, or to provide more specific items of data. Such paging devices are "active" devices in that they actually transmit a signal.
Cellular telephones are also "active" devices in that whenever a cellular telephone is switched on, at regular predetermined intervals, it transmits a brief signal, intended to be received by the closest cell transmitter/receiver, so that the central computers operating the cellular telephone system may be aware of the identity of the cell in which each telephone that is currently switched on is actually located. Of course, when a mobile telephone is actually being used to make a call, it is also transmitting signals.
There is reason to believe that the human body may be damaged on exposure to high intensity radio frequency radiation. It is radiation of this type that is given off by an active paging device and by a cellular telephone. A particular difficulty may be found to exist with cellular telephones which, when in use, are held very close to the head. Signals transmitted by the cellular telephone must be of a sufficient intensity to be received by the transmitter/receiver of the cell. Consequently, there is a substantial risk that the degree of radiation applied to the head of a person using the cellular telephone may be such that, at least in the long term, damage to the brain may be caused.
It has been proposed to provide a shielding device for use with an active pager and for use with a mobile telephone, but prior proposed shielding devices suffer from various disadvantages and drawbacks.
One disadvantage is that when the shielding device is operational, the shielding effect that is provided may be so great that the communication unit may not be able to receive signals transmitted to it by the appropriate transmitter/receiver.